This invention relates generally to turbine vanes in gas turbine engines and a bicast assembly of a ceramic vane to a to metal housing.
It has long been recognized that the efficiency and performance of gas turbine engines could be improved by increasing the temperature of the gas through the turbine section. Historically, these temperatures have been limited by the materials, usually high temperature steel or nickel alloy, used to form the first stage stator vanes. To permit higher gas temperatures it has been proposed to form the first stage stator vanes from a high density, high strength, silicon nitride, or silicon carbide ceramic which can withstand higher temperatures than steels or nickel alloys. However, the use of ceramic stator vanes necessitates a ceramic-to-metal interface at which the difference in thermal expansion between the ceramic vane and the metallic support structure must be accommodated so that the vanes remain fixed relative to the structure despite temperature changes in the gas. It also necessitates that the ceramic-to-metal interface prevent the vanes from twisting when subjected to aerodynamic loads.
Bicasting is a method used to form turbine stators. This method includes casting the shroud around the tip and root edges of prefabricated vanes. The advantage to bicasting is that the vanes and shroud can be formed from materials having different compositions. Kington et al. U.S. Pat. No. 5,290,143 discloses one such bicast stator.
A problem that has arisen in casting ceramic vanes to metal casings or housings is the need to accommodate the large thermal growth mismatch between the ceramic and the metal. If not dealt with properly, this mismatch will induce stress levels in the vanes and casing that may lead to a failure of a part during service.
Accordingly, there is a need for a cast assembly of a ceramic vane to a metal housing that can accommodate thermal growth mismatch.
An object of the present invention is to provide a cast assembly of a ceramic vane to a metal housing that better accommodates thermal growth mismatch.
The present invention achieves this object by providing a bicast assembly comprising a ceramic vane with a metal housing. The ceramic vane has dovetailed shaped grooves along it edges that extend from the vane""s leading edge to its trailing edge. The housing is comprised of two spaced apart walls each having dovetailed protrusions for mating with the dovetail grooves in the vane""s edges. Upon the casting of the walls to the vane at least one surface of each dovetail groove comes in contact with at least one surface of the dovetail protrusion.
A crushable coating is disposed between the vane""s edges and the dovetail protrusions.
The dogbone shaped connectors between the vane and the walls generates a very low stress condition in the ceramic vane. This allows for local thermal growth differentials associated with the casting process, engine operation and room temperature while at the same time ensuring positive contact of the vane at engine operation to prevent vibration or other functional problems associated with loose vanes.
These and other objects, features and advantages of the present invention, are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.